Conditioning of a gas stream, such as air, generally involves the removal or addition of moisture and the increase or decrease of temperature to make the gas stream suitable for its intended environment. For air conditioning in warm weather, this typically involves dehumidifying and cooling the air to comfortable levels.
Current dehumidification technology is commonly based on the conventional, refrigerant vapor compression cycle (hereinafter referred to as DX technology) or on desiccant substrate capture technology (hereinafter referred to as DS technology). DX technology requires cooling humid supply air, such as the air within a room and/or outside air, to the water vapor condensation point, with external heat rejection on the compression side. This usually requires the supply air to be cooled below comfortable temperatures and, thereafter, either reheated or mixed with warmer air to raise its temperature to an acceptable level before directing it into the space being dehumidified. Twenty to thirty-five percent (20-35%) of the energy expended in cooling the high humidity air is utilized to remove the latent heat from the air (the heat of condensation associated with water vapor condensation). Cooling and dehumidification of the air are thus coupled. That makes it impossible to independently control comfort parameters, making the DX cycle less efficient, from an overall system perspective, than a technology that would allow independent control of sensible and latent heat.
In applications where the outside air has both high humidity and temperature and the functional use of the interior space generates high water vapor levels (e.g., populated convention halls, exercise rooms, school buildings, etc.), it may not be possible for the DX technology to maintain the air introduced into the interior space at the correct humidity and temperature for maintaining comfort. The air delivered is cool but “muggy”, since further cooling to remove additional water would result in the air being uncomfortably cool.
In stand-alone dehumidification using a conventional compression cycle, heat reject is in direct contact with the room air. As a consequence, the room air becomes more comfortable from a humidity side, but may be less comfortable (too warm) from a temperature parameter consideration. Again the comfort parameters are coupled.
DS systems are generally applied in central air, ducted systems. Water vapor is captured by capillary condensation on a solid phase substrate containing pores of the appropriate size (typically less than 100 Angstroms) to cause capillary condensation. The capture process is efficient and rapid. However, removal of the water vapor from the pores, wherein the intrinsic vapor pressure of the water is lowered in correspondence with the Kelvin equation, requires energy input. It also requires removing the substrate from the high humidity air stream and placing it in an exhaust, water reject stream, before adding the re-evaporation energy. Alternatively, the substrate may remain fixed and the treated air and exhaust streams flow directions interchanged as is done in a parallel bed, desiccant drier system.
In these DS systems, the re-evaporation energy is the latent heat of condensation plus the heat of adsorption of the water vapor in the substrate pore material. It is important to note that DS technology requires, in steady state operation, the addition of this energy at a rate equal to or greater than the latent heat of condensation of water in the desiccant substrate. That is, the water vapor reject power input must exceed the equivalent latent heat of condensation power. After water removal from the desiccant substrate, the substrate must be re-cooled to the water capture temperature range of the substrate. As a consequence, some of the sensible heat of the subsequent cooling system (e.g., a DX cooling system) must be utilized in treating the DS substrate rather than for cooling the now dehumidified air.
The advantage of DS technology is that humidity levels in the outside air and/or recirculated air can be adjusted independently of the subsequent cooling step. The disadvantage is the requirement to move the substrate and treated air stream relative to each other for capture and rejection of the water vapor. This requires moving a large substrate through a sealed system, or, in a parallel bed DS system, requires complicated valving and valve cycling to move the humid air stream and an exhaust stream alternately across the desiccant beds. Again, application in typical stand alone, non-ducted room-type dehumidifiers is difficult if not impossible.
U.S. Pat. No. 6,539,731 discloses another type of dehumidification technology that utilizes a porous wall separating humid air from an osmotic fluid. The porous wall includes a capillary condenser layer and an osmotic layer. The capillary condenser layer is formed from ceramic materials having pores sized small enough to permit water vapor within the air to condense into liquid form. An osmotic driving force, resulting from a water concentration gradient, transports the condensed water through the capillary condenser layer and the contiguous osmotic layer, and into the osmotic fluid. The osmotic layer prevents substantially all of the osmotic fluid from entering the capillary condenser. This type of device is effective in promoting water transport at fluxes in excess of 1 liter/square meter-hour. Under some circumstances, however, the pore structure of the capillary condenser can become unstable and degrade over a relatively short period of time in a humid environment. In addition, the capillary condenser is typically made from a rigid material and is therefore limited to those applications where a rigid body is acceptable; i.e., the rigid capillary condenser cannot be used in those applications where a flexible device is required.
What is needed is a process and an apparatus for dehumidifying a gas stream that overcomes the deficiencies of the prior art.